pH up and pH down are tank controllers that will adjust your solution to the desired pH.
pH is a measure of the acidity or alkalinity of a solution. The pH scale ranges from 1 to 14.
pH 7.0 is considered neutral and is based on pure water.

Solutions with a pH less than 7 are considered acidic, while those above 7 are considered basic (alkaline). pH effects nutrient uptake for most plants. The optimum pH range for plants is from 5.8 to 6.5.

They are available as an RTU and concentrate.